The general aim of this project is to understand how the activities of the Jun and Fos families of transcriptions factors implicated in the proliferative response of mammalian cells to growth factors are regulated by interaction with other proteins. The specific aims are 1) to characterize the interactions of recently discovered proteins with Jun or Fos family members, 2) to determine the functional effects of the interactions, and 3) to identify other proteins that regulate Jun and Fos. The recently discovered proteins that interact with Jun or Fos were identified in the yeast two hybrid system. One is a DNA-binding homeodomain protein, another a novel leucine zipper protein, and a third is the mammalian homolog of the yeast transcriptional regulator SUG1. An early objective is to determine by mutagenesis the structural requirements for the interaction of each protein with the relevant domain of Jun or Fos. In the case of the first two proteins, effects on the transcriptional activities of Jun or Fos in transfected cells have been demonstrated, and effects of the third will be tested. The structural requirements for the transcriptional effects will then be correlated with the requirements for interaction with Jun or Fos. In the case of mammalian Sug1, anti-Sug1 antibodies will be used to determine if Sug1 is present in cell nuclei as a multiprotein complex, as recently reported for S. cerevisiae. If so, the complex will be purified and its interaction with c-Fos and related transcription factors will be explored. Finally, in collaboration with Jeff Boeke's laboratory, other proteins that regulate the activities of Jun and Fos will be identified using a sensitive two hybrid yeast system developed in Dr. Boeke's laboratory.